How to calculate the addition and subtraction of rational numbers in junior high school mathematics? Seek a detailed explanation

Add:

① Positive number+positive number = positive number

② Negative number+negative number = negative number

The absolute value of a positive number is greater than that of a negative number. Then:

③ Positive number+negative number = positive number

The absolute value of a positive number is less than that of a negative number. Then:

④ Positive number+negative number = negative number

Subtraction:

① positive number (large)-positive number (small) = positive number

② Positive number (small)-positive number (large) = negative number

③ Positive number-negative number = positive number

④ Negative number-positive number = negative number

⑤ Negative number (large absolute value)-Negative number (small absolute value) = negative number

6. Negative number (small absolute value)-negative number (large absolute value) = positive number.

Note: the absolute value is to change the symbol in front of this number into a plus sign.